1. Field of the Invention
The invention relates to a photoacoustic detector for measuring fine dust in gas.
2. Background Description
Photoacoustic measuring methods, i.e., measuring methods in which the substance being tested, preferably a gas or a solid object surrounded by a gas, is irradiated by a light source and heated by absorption, are well suited for precise measurements of the concentration of absorbing substances. The heating produces an expansion of the surrounding gas. If the heating, and thus, the expansion take place periodically, a sound wave is generated, which can be measured by a sound pressure sensor.
The measurement of fine dust in the air has played a role for years in the monitoring of air quality. In addition, efforts are also being made to determine the fine dust content of exhaust gases, wherein in particular the content of soot particles is of interest. Photoacoustic measuring methods have been known for this purpose for some time. Due to the increased expense in terms of equipment, however, they have not gained acceptance in the past. Possibly playing a role in this connection is that, in the monitoring the ambient air, as a rule average values over longer periods of time, for instance, averages over a half hour or an hour, are needed. It lends itself here to guide the air through a filter and measure the clogging thereof. The possibilities for automation are limited, however. In the case of the adjustment and inspection of combustion engines, for example, current values, rather than longer average values, are of interest. Of crucial interest in the measurement of fine dust particles is also the size of the fine dust particles. The size of the fine dust particles is of crucial importance in their harmful effect on health.
The suitability of the photoacoustic effect for measuring fine dust particles has already been proven by scientific studies by A. Petzold, R. Niessner “Photoacoustic sensor for carbon aerosols.” Corresponding devices can also be acquired commercially, for instance, AVL Micro Soot Sensor, http://www.avl.com. However, the size of the fine dust particles cannot be determined therewith.
Determining particle sizes in a liquid with the aid of a photoacoustic measurement is known from U.S. Pat. No. 4,722,602. The photoacoustic pulse produced is used for this purpose.
Inferring the size of particles, preferably particles in liquids, from the amplitude of the generated acoustic wave is known from U.S. Pat. No. 5,178,836.
Determining the size of particles in a liquid photoacoustically from the intensity and the phase shift is known from JP 59126933.